Backlights are used to illuminate liquid crystal displays (LCDs). LCDs with backlights are used in small displays for cell phones and personal digital assistants (PDA), as well as in large displays for computer monitors and televisions. Typically, the light source for the backlight includes one or more cold cathode fluorescent lamps (CCFLs). The light source for the backlight can also be an incandescent light bulb, an electroluminescent panel (ELP), or one or more hot cathode fluorescent lamps (HCFLs).
The display industry is enthusiastically perusing the use of LEDs as the light source in the backlight technology because CCFLs have many shortcomings: they do not easily ignite in cold temperatures, require adequate idle time to ignite, and require delicate handling. LEDs generally have a higher ratio of light generated to power consumed than the other backlight sources. So, displays with LED backlights consume less power than other displays. LED backlighting has traditionally been used in small, inexpensive LCD panels. However, LED backlighting is becoming more common in large displays such as those used for computers and televisions. In large displays, multiple LEDs are required to provide adequate backlight for the LCD display.
Circuits for driving multiple LEDs in large displays are typically arranged with LEDs distributed in multiple strings. FIG. 1 shows an exemplary flat panel display 10 with a backlighting system having three independent strings of LEDs 1, 2 and 3. The first string of LEDs 1 includes 7 LEDs 4, 5, 6, 7, 8, 9 and 11 discretely scattered across the display 10 and connected in series. The first string 1 is controlled by the driver circuit 12. The second string 2 is controlled by the driver circuit 13 and the third string 3 is controlled by the driver circuit 14. The LEDs of the LED strings 1, 2 and 3 can be connected in series by wires, traces or other connecting elements.
The strings 1, 2 and 3 are controlled by a controller by way of drivers 12, 13 and 14 respectively. FIG. 2 shows a prior art controller 20. FIG. 2 specifically shows the controller 20 for controlling string 1, by way of example. However, the controller 20 can also be used to control strings 2 and 3. The controller 20 includes an error amplifier 22, a continuous time loop compensation circuit 24, summation node, a local feedback loop 27 and a system feedback loop 28. The controller 20 provides a real time analog control of the string 1. The error amplifier 22 receives a reference voltage VREF as an input. The error amplifier 22 also receives a feedback signal VFB from the LED string 1 as an input by way of the system feedback loop 28. One of ordinary skill in the art will appreciate that the system feedback loop 28 includes the capability to scale the feedback signal such that the error amplifier 22 can properly compare the feedback signal with VREF.
Typically, VREF is indicative of the desired drive voltage that should be provided to string 1 to cause a desired current to flow through string 1. The error amplifier 22 compares the VREF with the feedback voltage VFB, which can be the sensed voltage indicative of the actual current flowing through string 1, and provides a result of the comparison to the loop compensation block 24. The output of the error amplifier 22 represents the correction that must be made to the drive voltage of string 1 to cause the desired current to flow through string 1. The error amplifier 22 continuously receives the feedback signal in real time from string 1 and provides the correction signal to the loop compensation block 24.
The loop compensation block 24 provides the proper drive voltage to string 1 by way of the driver 12, in response to receiving the correction signal from the error amplifier 22. The loop compensation block 24 thus continuously adjusts the drive voltage for string 1 in real time. FIG. 2 shows that the loop compensation block 24 is coupled to the driver 12 by way of the summation node (Σ). The summation node receives the output of the loop compensation block 24 as an input. The summation node also receives a feedback signal from string 1 by way of the local feedback loop 27. The feedback signal received by way of the local feedback loop 27 can be representative of, for example, the noise in string 1. The feedback signal received by way of the local feedback loop 27 can also be representative of, for example, an open circuit condition or a short circuit condition caused by string 1 or some other part of the display circuit. The summation node can provide for a quick adjustment to the driver 12, including shutting down the driver 12 output during abnormal conditions, depending on the circuit design and goals.
FIG. 3 shows another prior art controller 30 for controlling string 1. The controller 30 includes an analog to digital (A/D) converter 31, an analog to digital (A/D) converter 33, a digital signal processor (DSP) 32, a digital to analog (D/A) converter 34, and a buffer 35. The controller 30 provides for digital control of string 1 by way of the driver 12. The A/D converter 33 receives a reference signal VREF as an input. Typically, VREF is indicative of the desired voltage that should be used to drive string 1 in order to cause a desired current to flow through string 1. The A/D converter 33 converts the analog VREF signal into digital data and provides the digital data to the digital signal processor (DSP) 32.
The A/D converter 31 receives a feedback signal VFB by way of the system feedback loop 38. VFB can be the sensed voltage representative of the current flowing through string 1. The A/D converter 31 converts the analog VFB signal into digital data and provides the digital data to the DSP 32. The DSP 32 can be programmed to use the digital data received from the A/D converter 31 to determine the drive voltage for string 1. The DSP 32 can make intelligent decisions about controlling string one because it has access to various programs, comparison algorithms, look up tables and the like, that provide for consideration of various real-time system variables (e.g. ambient temperature) and non-real time system variables in the decision making. The DSP 32 provides the digital data related to the selected drive voltage to the digital to analog (D/A) converter 34. The D/A converter 34 converts the digital data into an analog drive signal, and provides the analog drive signal to the driver 12.
FIG. 3 shows that the DSP 32 is coupled to the driver 12 by way of the buffer 35. The buffer 35 can be used to store and hold the analog signals received from the D/A converter 34. The buffer 35 can include, for example, banks of storage capacitors for storing analog signals. The buffer 35 can be used to convert the outputs of the D/A converter 34 into smooth signals, for example, square waves, for driving string 1. FIG. 3 also shows that DSP 32 receives a feedback signal from string 1 by way of the local feedback loop 37. An analog to digital (A/D) converter 36 converts the analog feedback signal into digital data. The feedback signal received by way of the local feedback loop 37 can be representative of, for example, the noise in string 1. The feedback signal received by way of the local feedback loop 37 can also be representative of, for example, an open circuit condition or a short circuit condition caused by string 1 or some other part of the display circuit. The DSP 32 can provide for a quick adjustment to the driver 12, including shutting down the driver 12 output during abnormal conditions, depending on the algorithms and programs included in the DSP 32.
The controllers 20 and 30 shown in FIGS. 2 and 3 have many drawbacks. Controller 20 operates singularly according to the natural properties and characteristics of the analog circuit components, such as resistors, capacitors and inductors, and cannot be programmed to perform intelligent operations. Controller 20 is also subject to noise and delays that are inherent in analog circuit components. Controller 30 is subject to a relatively slow start up and boot up periods, inherent in digital systems. Also, the analog to digital to analog conversions and the digital signal processing result in time delays, and, as a result, real time control may not be available for many applications of controller 30. Furthermore, to program, debug or repair the DSP 32 during operation of the controller 30, the DSP 32 freezes the digital data provided to the D/A converter 34. That results in D/A converter 34 continuously providing the same output signal to the driver 12 during the freeze period. The feedback signal received by way of the system feedback loop 38 is ignored during the freeze period. That is undesirable.
The present invention provides a low power, high speed controller with a quick start-up period that can be programmed for intelligent decision making and can also perform real time operations.